Magnetic loop antennas (also known as small transmitting/receiving loops) are small compared to other antennas for the same wavelength. A magnetic loop is one in which the current amplitude is constant round the circumference, and it is therefore small enough to avoid a significant standing wave in the current distribution. Small transmitting loops are typically smaller than 1/4 wavelength in circumference at the intended frequency of operation. Since full-size antennas for shortwave communication can be very large -- sometimes several hundred meters in size -- the magnetic loop's ablility to operate with roughly comperable efficiency with much smaller size gives it some dramatic advantages, especially for mobile use and military use. Conceptually, the magnetic loop can be thought of as a high Q tank circuit having very large diameter to length ratio to facilitate the magnetic "leakage" that makes it effective as an antenna.
Analysis of the design by antenna professionals, confirmed by controlled experiments, has shown that high radiation efficiency is not always obtained, and that the main advantage of the antenna is its high-angle radiation and compact size.
Usually a capacitor is used to "enlarge" the antenna by tuning it to resonance in a parallel L-C circuit. The disadvantage of this method is the low bandwidth of the antenna (high Q) which limits efficient operation to a narrow frequency range. A high Q can also be advantageous, however, since well-tuned magnetic loops function best within a narrow frequency range, they reject noise from other sources. This reduce the noise level as compared with wider-bandwidth antennas. However, because all transmitted signals require a finite bandwidth, the high Q of magnetic loop antennas can produce problems when used for higher bandwidth applications.
As a result of the narrow operating bandwidth of the antenna, if the frequency of operation is changed, the antenna must be retuned by changing the value of the antenna's tuning capacitor. (Bandwidth is the usable frequency range of an antenna in relation to the area of desired operation and relates to the concept of Standing Wave Ratio or SWR.) When the antenna is operated outside of its bandwidth, the energy from the transmitter is reflected back from the antenna, down through the feedline, and back to the transmitter. When the reflected power exceeds about a 2.5:1 power reflection ratio (that is, when too much energy being reflected from the antenna back into the feedline) the antenna does not maintain the desired performance characteristics. This condition can affect the antenna's ability to transmit radio energy from the transmitter to the antenna.
In addition to narrow bandwidth, magnetic loops typically have very low radiation resistance, often one or two orders of magnitude less than a full size antenna such as a dipole, and only a fraction of an ohm. Efficiency thus depends on low-loss construction, typically use of thick conductors, and low loss air, mica, or vacuum dielectric capacitors.
In addition to the high currents resulting from the low radiation resistance, high voltages appear across the tuning capacitor when the loop is used for transmission; a kilowatt transmitting loop can have currents of the order of 100 Amperes and voltages at the capacitor of several tens of kV.
Magnetic loops are often fed with 50 ohm coaxial cable connected across a smaller coupling loop that is 1/5 to 1/8 the size of the antenna. This feed loop provides an impedance match to the loop's low feed resistance over the widest frequency range when it is located on the side of the antenna opposite the tuning capacitor. A less common feed system breaks the tuning capacitor into a two series capacitors with the feed across one of them.
The magnetic loop antenna is an old design which in limited use because of its low radiation efficiency and narrow bandwidth. However many military, commercial, and amateur radio operators still use them today because of the advantages conferred by its small size and easy transportability. The magnetic loop was widely used in the Vietnam war due to its high portability.
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